1. Field of the Invention
The present invention generally relates to particle optical lenses, such as electron beam lenses, and more particularly, to a variable curvilinear optical axis for such lenses to correct for aberrations.
2. Description of the Prior Art
In light optics, it is possible to maintain low aberrations and distortions while utilizing a significant portion of the lens area for imaging. In electron optics, however, it is not practical to correct the field of a lens to the same extent as with light optics. This is because the electron optical lens is actually a magnetic field rather than a piece of optical glass, and it is not possible to shape the magnetic field to any desired shape or to the same precision that a piece of glass can be formed. The magnetic field must, after all, satisfy LaPlace's equation within the lens. This problem is typically overcome in electron optics by making the lens as large as possible, or practical, relative to the optical field of view while keeping the focal length as short as practical for the given application. Making the lens large relative to the application has the effect of approximating the field shape of an "ideal" lens, much the same as is done in light optics when a small portion of a large spherical surface is used to approximate a parabolic surface. With probe forming systems, this means staying as close to the lens center or optical axis as the off-axis distortions and aberrations will allow. It is usually the case that the on-axis lens errors are smaller than the off-axis errors and that the off-axis errors increase with the square or cube of the distance off axis. If higher order error terms are considered, than the errors will increase as the higher powers of the terms.
It is possible to deflect an electron beam at very high speeds either electrostatically or magnetically or a combination of both. Thus, any point can be addressed within a relatively large defection field in very short times (on the order of microseconds or even nanoseconds). The final location of the beam can also be corrected during deflection by modifying the deflection address according to some predetermined distortion map acquired during system calibration and/or wafer registration. This is a common practice, but it only corrects the landing position of a single ray or small bundle of rays defining a point which is transferred from the object plane to the image plane. Any lens errors will still distort the local region about this central ray. A common practice to correct some of this local image distortion is to refocus and apply a stigmation correction to the off axis beam. The further the beam is deflected off the central axis, the greater the deflection aberrations will become. At some point, further deflection is rendered unusable due to excessive lens aberrations that are not correctable by methods known in the art. The inventions disclosed in U.S. Pat. No. 4,859,856 for a Variable Axis Lens (VAL) and U.S. Pat. No. 4,544,846 for Variable Axis Immersion Lens (VAIL) used a technique of subtracting a planar field from the lens' radial field. This planar field is everywhere parallel to the radius connecting the central z-axis and the point to which the beam is deflected. The term "planar" is used to refer to a field, such as that resulting from a deflection yoke (typically of either a Saddle or Toroidal configuration) where the field in any z plane is uniform, but the magnitude of the field may vary according to a smooth function of z as one moves from z-plane to z-plane. As described in the above inventions, the strength of the planar field subtracted from the lens radial field is proportional to the first derivative with respect to axial position, z, and to the distance the lens field is to be shifted in the radial direction. The typical method of applying the planar field is by means of a deflection yoke sized and positioned to match the negative of the first term of the radial field of the lens. This has the effect of shifting the optical axis laterally with the deflected beam so that to the beam it appears as though it is still on the optical axis. By this method, the off-axis errors of the lens and deflection system can be greatly reduced.
This technique is not a perfect solution because it corrects only to the first order, which is the greatest part of the errors; however, this approach also assumes that the effective axis of the lens remains a nearly straight line shifted parallel to the geometric axis of the lens. The electron beam is deflected prior to entering the field of the lens such that the beam ends up traveling coincident to the shifted axis as it travels through the lens. This is done so that the electron beam does not deviate substantially from the shifted axis and therefore does not incur any errors greater than is allowed by the system error budget. Such an approach requires a considerable spacing between lenses and deflection yokes; however, in a practical system design, some overlap of the beam deflection and lens will occur.
The separation of the lens and beam deflection yokes cannot increase without penalty. The longer the path length of the electrons, the more Coulomb interaction between electrons will occur. As a result of the Coulomb interaction it would therefore be desirable for the beam deflection and lens to be "fully" overlapped in order to keep the optical path length that the electron travels to a minimum. This Coulomb interaction creates additional errors which add to the lens and deflection errors.
The problem, therefore, is how to achieve the largest electron optical field of deflection with the smallest errors possible in the shortest optical path length possible.